1. Field of the Invention
The present invention relates generally to a backlight unit installed in a Liquid Crystal Display (LCD) device and, more particularly, to a Light Emitting Diode (LED) backlight unit without a Printed Circuit board (PCB), in which light source modules, that is, LED packages, are directly mounted on a chassis.
2. Description of the Related Art
A backlight unit is a device that is installed in an LCD using the principle in which liquid crystals change their molecular arrangement according to applied voltage, and provides light and illuminates a screen from behind. Although backlight units to which one or more cold-cathode tubes are applied were mainly used, backlight units to which LEDs are applied currently attracts attention due to their advantages with respect to life span, brightness, color reproducibility, etc.
Unlike cold-cathode tubes, LEDs require substrates when they are used as light sources. Since such LEDs emit large quantities of heat while radiating light, metal core substrates (metal core printed circuit boards), having an excellent heat dissipation characteristic, have been used.
Although metal core substrates have an excellent heat dissipation characteristic, they are very expensive. Accordingly, the high cost of the metal core substrates is one of the principal factors that decrease the cost competitiveness of the backlight units formed of the metal core substrates. As a result, there is a trend toward the use of relatively inexpensive epoxy resin insulating substrates. An example of a conventional backlight unit in which LEDs are mounted on such an insulating substrate is illustrated in FIG. 4.
As illustrated in FIG. 4, a backlight unit 200 includes an insulating substrate 210, a plurality of LED packages 230, and a chassis 250.
Circuit patterns 211 and 212 are formed on the insulating substrate 210 by coating an epoxy resin FR4-core with a copper foil and etching the copper foil.
Each of the LED packages 230 is mounted such that an LED chip 231 is directly connected to one LED electrode 232 and is wire-bonded to the other LED electrode 233.
The LED chip 231 and the LED electrodes 232 and 233 are placed within a plastic mold casing 234, and the casing 234 is covered with an epoxy resin lens 235.
The LED package 230 is mounted on the insulating substrate 210, and is electrically connected to positive and negative electrodes, that is, to the circuit patterns 211 and 212.
The chassis 250 is made of material having excellent thermal conductivity, such as metal, and is placed beneath the insulating substrate 210, and a heating pad 270 is placed therebetween so as to provide electrical insulation and decrease contact thermal resistance.
In the conventional backlight unit 200 having the above construction, when the total thermal resistance is calculated, with the assumption that the thickness tsubstrate of the substrate 210 is 0.8 mm, the thermal conductivity Ksubstrate of the substrate 210 is 0.35 W/m ° C., the thickness theating pad of the heating pad 270 is 0.2 mm, the thermal conductivity Kheating pad of the heating pad is 1.00 W/m ° C., and the area A of the substrate 210 or the heating pad 270 is 36 mm2,
                              R          total                =                ⁢                              R            substrate                    +                      R                          heating              ⁢                                                          ⁢              pad                                                              =                ⁢                              (                                                            t                  substrate                                ⁢                                  /                                ⁢                                  K                  substrate                                            +                                                t                                      heating                    ⁢                                                                                  ⁢                    pad                                                  /                                  K                                      heating                    ⁢                                                                                  ⁢                    pad                                                                        )                    ⁢          A                                        =                ⁢                              (                                          0.8                *                                                      10                                          -                      3                                                        /                  0.35                                            +                              0.2                *                                                      10                                          -                      3                                                        /                  1.00                                                      )                    /                      (                          36              *                              10                                  -                  6                                                      )                                                  =                ⁢                  69.4          ⁢          °          ⁢                                          ⁢                      C            .                          /                                ⁢          W                    
As explained above, the total thermal resistance is very high. When the thickness of the heating pad 270 is 0.5 mm, a total thermal resistance of 77.88° C./W results.
As a result, the insulating substrate of the conventional backlight unit having the above-described construction exhibits very poor thermal conductivity, therefore it is difficult to effectively eliminate heat generated by the LED chips, with the result that the temperature of the LED chips still increases. Accordingly, the amount of light emitted by the LED chips decreases, variation in wavelength occurs, and the reliability of the LED chips decreases, thus resulting in a reduced lifespan.
Furthermore, it is required to use a substrate to provide the circuit patterns connected to each LED chip. This increases the price of the backlight unit as well as the total thickness thereof.
Furthermore, the heating pad is made of a high polymer material having appearance characteristics similar to those of a thin rubber plate. This not only increases the price of the backlight unit and but also requires close attention to be paid to an assembly process.